Hot melt pressure sensitive compositions are used widely in the adhesive industry due to their ability to adhere to a wide variety of substrates. Block copolymers are a preferred base polymer because of their elastomeric properties, their ability to be tackified, their compatibility with a wide range of tackifiers and plasticizers, and their high cohesive strength. Block copolymers are also popular because of the wide variety of grades available with varying molecular weights, styrene contents, coupling efficiencies and types of midblocks. The block copolymers most widely used have styrene endblocks, but the amount of styrene being present in varying amounts. Representative examples of midblocks most often encountered include isoprene, butadiene, ethylene/butylene and ethylene/propylene.
A primary property of a hot melt pressure sensitive composition is that it remains tacky at room temperature and can be adhered to substrates after it has completely cooled from its molten state. Block copolymers by themselves are inherently nontacky and it is necessary to use tackifying resins and plasticizers to develop the tack necessary for pressure sensitive adhesion.
Pressure sensitive adhesives are utilized for various applications including, but not limited to, labels, tapes, and packaging.
Block copolymers generally exhibit high cohesive strength resulting in adhesives that have high cohesive strength. While high cohesive strength is desirable for most applications such as for tapes and labels, this may not always be the requirement. Cohesive failure is a desirable property for such applications as seal/reseal in the packaging industry. This type of package is peelable and reclosable indicating that it's easy for a consumer to open initially, and it may be resealed later either through use of mechanical means or through an adhesive for instance. This type of package may be utilized for perishable goods.
The films utilized for manufacturing seal/reseal type packages are generally polymeric in nature and may comprise polyethylene, polypropylene, polyvinyl chloride, polyester, and acrylonitrile-methyl acrylate copolymer polymerized and/or mixed with butadiene as a terpolymer. The problem encountered with block copolymer based products for seal/reseal applications of this nature is that the adhesive generally remains on one side of the container or the other due to the fact that the internal strength of the adhesive is as high or slightly higher than the bond strength or peel adhesion to the substrate. This results in stringing of the adhesive as it has a tendency to remain on the package, but also to stay associated with itself. The adhesive therefore does not remain adhered to the substrate as intended, but strings in an attempt to remain with both resulting in what is known in the art as "adhesive confusion."
There is prior art that describes the use of a higher cohesive strength adhesive that remains associated preferentially with one side of the package and does not fail cohesively. This is an attempt to alleviate the problem of stringing of the adhesive but it does not, however, solve the problem of "adhesive confusion," as the internal strength of the adhesive is only slightly higher than the peel adhesion to the substrate.
U.S. Pat. No. 4,096,203 to St. Clair teaches a low cohesive strength hot melt adhesive employing either a styrene-butadiene-styrene(SBS) block copolymer or a styrene-isoprene-styrene(SIS) block copolymer. SBS and SIS block copolymer based adhesives are known to exhibit poorer thermal stability, and also exhibit an odor that is not as desirable for the packaging of perishables such as food.
The current inventors have found a solution to the aforementioned problems by employing hot melt pressure sensitive adhesives based on block copolymers having a high diblock content which fails cohesively during use.